In imaging applications, radiation detectors for detecting x-ray radiation usually consist of a plurality of detector elements often also referred to as pixels. Since the individual detector elements and the associated read-out circuitry are usually not entirely identical, the responses of the detector elements to incident radiation often vary between different detector elements. These varying responses particularly result from gain variations of the detector elements, which may stem from inhomogeneities in the photon conversion yield of the detector elements, losses in charge transport, charge trapping, or variations in the performance of the readout circuitry, for example. These variations can be parameterized by means of an effective area of the detector element. Therefore, these variations are also referred to as gain type or effective area type inhomogeneities herein.
In order to eliminate or reduce image artifacts resulting from effective area type inhomogeneities, a so-called flat-field correction may be carried out. In one implementation, flat-fielding may involve illuminating the radiation detector with homogeneous radiation intensity such that all detector elements would detect the same radiation intensity in an ideal radiation detector. Then, the flat-field correction may be determined on the basis of relative differences between the output signals of the detector elements and a reference, such as a mean output of all detector elements.
Such a flat-field correction provides good results for integrating radiation detectors and also for photon-counting detectors, when such detectors are illuminated with a low radiation flux. Photon-counting or spectral detectors allow for detecting individual incident photons and for determining their energies in accordance with certain energy ranges (often also referred to as energy bins). For instance, such detectors are used in spectral CT scanners utilized in medical and other applications, such as, for example, material testing, in backscatter x-ray scanners used in security applications and in other devices. In these applications, radiation detectors are often operated at a high radiation flux, and it is has been observed that that the flat-field correction does not lead to satisfactory results at high radiation flux. Rather, similar image artifacts as those resulting from effective area type inhomogeneities often re-emerge at high radiation flux also after a flat-fielding of the radiation detector at low radiation flux.
It has been found that this is due to inhomogeneities of the dead times of the detector elements. The dead time of a detector element corresponds to the time after a conversion of an incident photon into an electric signal during which the detector element is not able to unequivocally detect another incident photon. The dead times of different detector elements of a radiation detector usually also varies and, especially at a high photon flux, such variations cause the aforementioned artifacts, which cannot be eliminated by means of the flat-field correction.
WO 2013/144812 A2 discloses an imaging system comprising a photon-counting x-ray detector. The detector has an input photon count rate determiner that determines the input photon count rate from the detected output photon count rate measured on the basis of an output photon count rate to input photon count rate map. The map is generated in an air scan on the basis of a paralyzable detector model and assigns two candidate input photon count rates to each output photon count rate. For a detected output photon count rate, the input photon count rate determiner determines the correct input photon count rate particularly on the basis of a ratio of input photon count rates for detector pixels having different radiation sensitive areas or measured using different shaping times of a shaper assigned to a detector pixel.